1. Field of the Invention
The invention relates to an apparatus for optically scanning a radiation-reflective information plane, which apparatus comprises a diode laser supplying a scanning beam, an objective system for focussing the scanning beam to form a scanning spot in the information plane and for re-imaging the scanning spot on a composite radiation-sensitive detection system, and a composite diffraction element which is arranged in the radiation path between the diode laser and the objective system for deflecting the radiation beam reflected by the information surface to the radiation-sensitive detection system and for splitting said beam into a plurality of sub-beams forming a corresponding plurality of radiation spots on a corresponding plurality of detector pairs of the composite detection system.
2. Description of the Related Art
An apparatus of this type, which is in principle suitable for reading a pre-recorded optical record carrier and for optically recording such a record carrier, is known from U.S. Pat. No. 4,665,310 . In this apparatus the composite diffraction element in the form of a diffraction grating performs two functions for which otherwise two separate elements are required. Firstly the grating ensures that the radiation which has been reflected by the information surface and which traverses the objective system is deflected from the path of the radiation emitted by the diode laser, so that a detection system can be arranged in the path of the reflected radiation. Secondly, the grating splits the reflected beam into two sub-beams required for generating a focussing error signal, i.e. a signal containing information about the magnitude and the direction of a deviation between the focal plane of the objective system and the information plane. Each of the sub-beams is associated with a separate detector pair, the signal representing the difference between the output signals of the detectors of the same pair being a measure of the focussing of the scanning beam on the information plane.
In the said record carrier the information is arranged in accordance with information tracks. If the bounding line between the two sub-gratings extends parallel to the track direction, it is possible, by determining the sum of the output signals of each detector pair and subtracting these sum signals from each other, to form a signal containing information about the magnitude and the direction of a deviation between the centre of the scanning spot and the central axis of the information track to be scanned.
In order to obtain the desired beam-splitting, the diffaction grating in the known apparatus comprises two subgratings having the same grating period, whilst the grating strips of the first sub-grating extend at a first angle and the grating strips of the second sub-grating extend at a second angle, which is equal but opposite to the first angle, to the line separating the two sub-gratings. Since a diffraction grating deflects an incident beam in a plane transversely to the direction of the grating lines, the part of the beam which is incident on one of the sub-gratings will be given a different direction than the part of the beam which is incident on the second sub-grating.
As set forth in U.S. Pat. No. 4,665,310 the grating design described in this patent is based on a previously proposed composite diffraction grating. This grating comprises two sub-gratings in which the grating strips of the one sub-grating have the same direction as those of the other sub-grating, but in which the grating periods of the two sub-gratings are different. Since the angle at which an incident beam is deflected by a grating depends on the grating period, the part of the beam incident on one of the sub-gratings is deflected at an angle which is different from the angle at which the part of the beam which is incident on the other sub-grating is deflected.
Satisfactory experience has been gained with scanning apparatus provided with these gratings. However, it has been found that when using a grating a deviation in the generated focussing error signal may occur which, it is true, remains within the range of tolerance laid down for this signal, but leaves only little room for possible other deviations. The last-mentioned deviations may be caused by mutual movements of the optical components, and by varying settings in the electronic processing circuit.
As is known the wavelength .lambda. of the radiation beams emitted by diode lasers which are frequently used in practice may vary, for example due to temperature variations. The wavelengths of individual diode lasers, which have been manufactured by means of the same process at different points of time, may also mutually differ. A wavelength variation of the scanning beam results in a variation of the angles at which the sub-beams are deflected by the sub-gratings, resulting in a change of the positions of the radiation spots on the detector pairs.
To prevent these position changes from affecting the generated focussing error signal, it has already been proposed to arrange the separating strips of each detector pair in such a way that the displacement of the radiation spots due to the wavelength variations occurs along these separating strips. However, the varying intensity distribution of these radiation spots has not been taken into account.
When correctly focussing the scanning beam on the information plane and in the case of the correct, or nominal wavelength of this beam, the sub-beams from the diffraction grating form radiation spots on their associated detector pairs, which spots have intensity distributions which are symmetrical with respect to these detector pairs. When varying the wavelength of the scanning beam, not only the positions of these radiation spots change but these spots also become asymmetrically larger in the direction transversely to the separating strips because the focussing of the sub-beams with respect to the associated detector pairs changes, even in the case of a constant and correct focussing of the scanning beam on the information plane. Then the fact that each sub-beam originates from a grating covering only half the exit pupil of the objective system, so that these sub-beams are asymmetrical, starts to play a role. The magnification of a radiation spot occurring as a result of the wavelength variation is asymmetrical, so that the centre of the intensity distribution of a radiation spot performs a movement with a movement component transversely to the separating strip of the associated detector pair. In the case of a wavelength variation there is therefore a change of the difference signal of the detectors associated with a pair, which change is interpreted by the focus servo-system as a focussing error of the scanning beam with respect to the information plane. The focus servo-system then starts to "correct" in such a way that the scanning spot is no longer focussed on the information plane in an optimum manner.